A recent report released by the Australian PV Institute (APVI) reveals solar photovoltaic curtailment is reducing the solar output of some households by up to 20 per cent, with inconsistences in different regions raising issues of fairness for solar owners, writes Gavin Dennett.

Solar PV curtailment is the intentional reduction of power output from solar energy systems to the grid, allowing the management of voltage to prevent possible unsafe levels and grid overloading. However, this practice also limits the potential for a PV system owner to earn revenue through exporting excess solar.

The APVI report, “Curtailment and Network Voltage Analysis Study (CANVAS), a RACE for 2030 Project”, was a technical and social science study conducted by the Collaboration on Energy and Environmental Markets (CEEM) at the University of NSW, with industry partners AGL, SA Power Networks (SAPN) and Solar Analytics.

It found that on average the reduction in rooftop solar export to the grid due to curtailment is less than one per cent, but in some areas with low voltage networks, a small number of households recurrently lose up to 20 per cent of their generation.

The scoping study in South Australia looked into two solar inverter quality response modes for curtailment – tripping/anti-islanding/limits for sustained operation, and Volt VAr settings – and investigated losses for 1000 PV and battery sites, and 500 PV-only sites in metropolitan Adelaide.

“The results indicate that average curtailment loss is less than one per cent of the generation, and greater for PV-only sites,” says Dr Baran Yildiz, the lead researcher of the CANVAS project. “However, a small number of households lost up to 20 per cent of their generation, which clearly raises fairness concerns.”

Switching off solar

In 2021, energy market operators in South Australia were granted permission to remotely switch off rooftop solar panels when the grid is at risk of being overloaded. Western Australia followed suit in February this year.

Electricity generation is often curtailed to manage grid capacity, particularly when oversupply threatens to overwhelm it – a scenario on the rise as more solar is installed across Australia. There are also economic reasons, such as when prices go negative in the grid and energy providers, such as operators of wind and solar farms, choose to stop exporting.

According to the Australia Energy Market Operator (AEMO), it is estimated that by 2050 more than 20 per cent of renewable energy will be curtailed, amounting to around 50 trillion kilowatt hours, which is more renewable energy than Australia currently generates.

While South Australia and Western Australia have granted energy market operators permission to curtail solar rooftop, the practice of curtailment in another form is actually taking place in every Australian state and territory already, with rooftop PV owners often unaware of it.

When a household’s solar inverter senses too much voltage in the local grid, it automatically switches off the rooftop panels.

“This happens the majority of the day for some households,” says Dr Yildiz. “But if owners aren’t monitoring their systems and not looking at their apps, it may be a while before they understand what’s happening.”

Varying curtailment levels

There is a range of factors that influence the levels of curtailment on PV systems, including size, type, location and age.

Analysis of the Solar Analytics data of 500 sites in Adelaide indicates that lost energy due to anti-islanding curtailment – when the solar is disconnected to the grid due to very high voltages – is very low, with an average of 0.35 per cent generation being curtailed. However, some sites are significantly impacted with up to 20 per cent curtailment.

The proportion of days on which some curtailment occurs due to anti-islanding is relatively high, with 20 per cent of sites experiencing curtailment on at least 21 per cent of days during the 10-month study. This suggests that while anti-islanding curtailment impacts a small proportion of overall generation, it occurs quite frequently.

The average financial loss for owners of PV systems due to curtailment is less than $5 per year, but for people most severely impacted, losses are around $40 per year. A large proportion of the people participating in the APVI study had no previous knowledge of curtailment, with many admitting they find the practice off-putting and unfair, especially if they are doubly impacted financially through lost revenue from exporting to the grid, and paying for mains electricity to run household appliances during curtailment periods.

While the current fallout from the curtailment issue may be relatively small, it is expected to exponentially become a larger problem as uptake of household solar continues to climb.

The study outlines the need for consumer education campaigns, clauses in solar contracts, notifications about curtailment events, and household-scale modelling of the likely impacts of curtailment on electricity bills.